How the genetic code was cracked

some possible 3-letter codes
The structure of DNA, solved in 1953, set off a race to crack the genetic code. How do sequences of 4 nucleotides code for sequences of 20 amino acids? This coding problem lies at the heart of molecular biology. Physicist George Gamow of Big Bang fame contributed the first guess: Spaces between neighboring nucleotides might fit individual amino acids, directly templating protein assembly on the DNA. In Gamow's solution, each nucleotide must contribute to defining two amino acids–an overlapping code. The numerology looked good (there were exactly 20 possible combinations), but Gamow's solution turned out to be wrong: In 1957, Sydney Brenner devised a simple test that disproved this and all overlapping triplet codes. The true code was soon cracked based on beautiful frameshift experiments by Crick et al., and by analysis of proteins synthesized from artificial RNAs.
Supplements: Gamow's guess, Brenner disproves Gamow and all overlapping triplet codes, the decisive artificial RNA experiments

Elucidation of the RNA interference trigger

The discovery of RNA interference revolutionized the way we determine the role of a gene.  The gene silencing phenomenon has been shown since the early 1990s when introduction of sense or anti-sense RNA could cause a reduction of endogenous messenger RNA.  In 1998, Fire and Mello provided an explanation for the previously reported silencing effect.  Their seminal paper shows that it was not ssRNA that silenced the endogenous RNA, but in fact dsRNA.  So how did ssRNA cause silencing in previous reports?  It is believed that their ssRNA preparations were contaminated with complementary RNA!  Fire and Mello overcame this by extensively purifying their RNA.  Indeed,  they showed that ssRNA was consistently found to be 10 to 100 fold less effective than double stranded.  To this day biologists continue to make great strides in understanding the roles of genes due to this discovery.
Nobel prize article and image credit 

A defective protein causes a disease!

It is obvious now that defects in proteins, normally because of mutations in the DNA, cause many diseases, but it was not so evident in 1949.

Linus Pauling and his collaborators knew that only deoxygenated blood contains the sickle shaped erythrocytes (see picture) characteristic of sickle cell anemia, which lead them to the hypothesis that hemoglobin was involved in this problem.

They showed that hemoglobin from patients suffering from sickle cell anemia is different (has different electrophoretic mobility) to the “healthy hemoglobin”. In addition, they found that people with sicklemia, a less severe version of the disease, contain both forms of the protein. This was proof of a change in a protein causing a disease!
More important that the actual experiment, are the conclusions derived of it. Not only this was the beginning of “molecular medicine”, but the genetic discussion in the paper is groundbreaking.
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How does the environment affect evolution? Waddington's example of genetic assimilation

Conrad Waddington, throughout his long and varied career as a developmental biologist, was foundational to several aspects of modern evolutionary theory, such as epigenetics, developmental canalization, and genetic assimilation. One of the great puzzles of evolution is how organisms can become so specifically and heritably adapted to their environment. Random genetic mutations can sometimes serve as the sole explanation, but not always. Through several rather cleverly simple experiments, Waddington demonstrated that phenotypes elicited by a specific environmental cue (such as heat shock or ether treatment) could be "assimilated" into the genotype. This means that the phenotype could eventually be expressed even if the corresponding cue was absent. He argued that this phenomena, which he witnessed in Drosophila in less than 30 generations, could play a powerful and vital role in evolution.
Background paper: Genetic assimilation of cross-veinless phenotype (1953)

Rats with an intense social life do not become drug addicts.

Between 1950 and 1975 many studies had shown that rats that have had morphine previously administrated become addicted to the drug, since later on they choose to drink a morphine solution when water is also available.

Bruce Alexander had a problem with those studies as those rats had been kept in “small, solitary metal cages” which, he thought, could influence the results. He therefore designed a Rat Park: an open-topped cage with sawdust on the floor and multiple toys (including a climbing pole) and friends to play with (see image).

He performed experiments to test morphine addiction in rats that had been either isolated or living in Rat Park and saw that social rats did not become addicted to morphine. He concluded that it was the “spatial confinement, social isolation, and stimulus deprivation” what made them drug addicts rather than the drug itself.

Barbara McClintock discovers "jumping genes"

Transposable elements (TE) are DNA sequences that “jump” from one location in the genome to another.  McClintock’s work not only showed that sequences can move, but also that this movement across the genome can create and reverse mutations as well as alter genome size, all during various stages of cell development.  McClintock conducted standard genetic self-breeding experiments causing broken chromosomes and noted unusual color patterns in the offspring, to understand the cause of this variety she compared the chromosomes of each generation with that of the parent and found that certain sections of the chromosomes had switched their position.  At first her discovery was met with skepticism because it went directly against the popular theory at the time that genes were fixed in their chromosomal position but McClintock’s work was rediscovered through work in bacteria a decade later and earned her a Nobel Prize in 1983.
Supplementary paper:  Junk DNA as an evolutionary force

Chicken or the Egg?

Chicken or the Egg? DNA or proteins? This age old question would still evoke debate if not for Nobel laureates Sidney Altman and Thomas Cech. Their individual works on catalytic Ribonucleic acid (RNA) showed there existed a biomolecule that could simultaneously be genetic material and enzymes. Thus neither the chicken nor the egg came first.

While studying the RNA of Tetrahymena thermophila, Thomas Cech found unprocessed RNA could self-splice. An important thing to note is that Thomas Cech made his discovery by pursuing a negative control. Rather than disregard the phenomena he was observing, Cech further investigated the case. His pursuit led him to become the first to show that these biomolecules were autocatalytic and not mere carriers of genetic information. This experiment is great because Cech sought answers for the data he collected rather than just sought data for his question: a fundamental concept oft forgotten in our pursuit of science.

Lenski "replays" evolution in longterm E. coli populations

Richard Lenski's longterm evolution experiments on E. coli are a hallmark example of evolutionary biology. Lenski and colleagues have maintained 12 parallel lines of E. coli for 50,000 generations now. Initially, these E. coli populations were founded by clones, and over decades, researchers have watched evolutionary dynamics on a scale observable in real time.

This particular paper, published in 2008, describes the acquisition of a novel phenotype - the ability to metabolize citrate in addition to glucose as an energy source.

Lenski's experiments on the evolution of citrate use are particularly elegant for the following reason: The lab maintains frozen samples of the E. coli populations at time points throughout the history of the populations. These samples are not growing (and therefore not mutating) while frozen, but can be pulled from the freezer and reconstituted. This allowed researchers to go back to previous timepoints in the evolution of this phenotype and "replay evolution" to see if the same phenotypes arise again ...

Are evolutionary changes selection-driven or are they random?

In 1943, Salvador Luria and Max Delbrück designed a simple, easy to perform experiment to answer a question that had caused many debates within the scientific community: do evolutionary mutations arise due to the stimulus that causes the selective pressure? Small E. coli cultures were grown until saturation and, then, were plated in selective media (with bacteriophage T1, which infects and kills E. coli). A small number of colonies showed up in each plate, coming from a mutant resistant to T1 infection. They predicted that, if the mutants were caused by the presence of the virus, there would be, roughly, the same amount of colonies in each plate (see figure). But that is not what they saw. They got a wide disparity in the number of colonies per plate, which led them to the conclusion that the mutations were developed before contacting the virus!
p.s.1: The Math/stats part of the paper is brutal, you should get into it at your own risk
p.s.2: A recent Microbiology discovery may make us expect the opposite results.

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How DNA synthesizes itself

After the structure of DNA was elucidated by Watson and Crick, one of the next burning questions was how is it replicated? The aforementioned individuals contributed the first hypothesis: that DNA replication is semi-conservative. That is, each strand of DNA serves as a template for a newly synthesized strand. A second was the conservative hypothesis, that the entire DNA molecule serves as a template for a new DNA molecule. And finally, the dispersive hypothesis proposed by Max Delbrück argues that a mechanism exists that would break the strand every so often and attaches a new strand to the old one. To test this, Matthew Meselson and Franklin Stahl, in an incredibly elegant experiment published in 1958, grew E. coli first with 15N then with 14N and allowed to divide. They periodically extracted the DNA and centrifuged the DNA in a cesium chloride density gradient. The results were obvious. Through cell division, half of the DNA was replaced with new DNA, favoring the Watson and Crick hypothesis that DNA replication is semi-conservative.

How insulin was first isolated to treat diabetes

In 1921, Clinician Dr. Federick Banting had a failing medical practice and a burning desire to do research, but had no formal training.  He became fascinated with the role of insulin and the pancreas in diabetics.  One night he had an epiphany on how to isolate insulin-producing islets from the pancreas.  Armed with no credentials, he was able to convince Professor John Macleod to let him use his lab space and one of his medical students to test his theory.  Incredibly, within a few months they were able to revive a dog from diabetic coma.  Several months later they were successfully treating human patients.  Within two years of walking into a lab for the first time, Dr. Banting was awarded the Nobel Prize in Medicine and controversially so was Dr. Macleod who Banting did not think deserved credit.  The choice to award Macleod is still a topic of disagreement today.
Nobel Prize article and image credit

How do growing nerve cells find their targets?

Rita Levi-Montalcini (photo credit)
In 1949, Rita Levi-Montalcini noticed something unexpected. Her colleague Elmer Bueker had found that nerves would invade tumors that he had implanted into chick embryos. What attracted the nerves to tumors? Indeed, how did nerves ever find their normal targets? What Levi-Montalcini noticed: the nerves would invade not just the tumors, but also the tissues near the tumors–suggesting that the tumors might have been releasing a diffusible nerve growth factor, a postulated substance that could guide either nerve differentiation, growth or survival. Levi-Montalcini proved the existence of a nerve growth factor by culturing just tumors and ganglia in the same dish, finding that the nerves from the ganglia would connect to tumors even in vitro. Later, she purified the key protein, now called Nerve Growth Factor (NGF). NGF told us that the way nerves find their targets is unexpectedly adaptive–nerves grow just about everywhere, and they die off if they fail to find targets. 
A short review: Aloe, L. (2004) Rita Levi-Montalcini: the discovery of nerve growth factor and modern neurobiology. Trends Cell Biol 14:395-9. 

Some amazing historical background: An excerpt about her pre-NGF work done in makeshift home labs she set up hiding out in the hills during WWII, from her autobiography, In Praise of Imperfection. Open the excerpt in the right pdf viewer and you'll see some helpful notes in red.

Dynamic Instability of Microtubules

The discovery of the dynamic instability of microtubules was a milestone in cytoskeleton research. Prior to these studies, the prevailing theory about microtubule dynamics at the time was that polymers continuously lost subunits on one end and gained them on the other, called treadmilling. The authors of these studies, Mitchison and Kirschner, originally were not interested in studying this process. Instead, they set out to study how centrosomes affect microtubule polymerization. However, in doing some dilution experiments, they observed results that were incompatible with this theory. They shifted gears and turned their attention to studying these findings in more detail. What they found was that microtubules exist in two different, interconvertible states. That is, microtubules grow and shorten depending on the concentration of tubulin. They coined this process dynamic instability.

Wibbly Wobbly Timey Wimey...Stuff
In quantum mechanics, the act of observing changes the behavior of photons. Unobserved, photons exhibit an interference pattern. Photons will have a particle-like behavior no matter if they are observed before or after going through the double slit. It would appear they go back in time to select a singular path rather than oscillate between many quantum events of the wave equation when unobserved. Clever physicists attempted to trick the photons by studying the results of an entangled pair rather than its path. However, the mere knowledge of the intended behavior changed the photons' paths. An even crazier concept explored was the behavior of the unobstructed half of the photon pair would be identical to its twin that experienced path manipulation. Despite having a shorter distance to travel, the outcome of the unobstructed would always correlate with the later one. It was as if they time travelled in order to coordinate.
This paper was referenced for its use of Young's double slit. It is a simple optical analogy of the original design, but necessary to show that the first experiment used to discover wave-particle duality can also be used to perform a quantum eraser experiment.
Video link